1. Field of the Invention
The invention relates generally to the localized application of etching or solvent liquids for thinning or removal of material or layers of material such as required for the decapsulation of molded devices and microcircuits and more particularly to an apparatus and process for decapsulation of devices wherein the device is unattacked and very clean so that it is highly suitable for internal visual inspection such as during routine destructive physical analysis or failure analysis.
2. Description of the Prior Art
Semiconductor devices are commonly encapsulated with a plastic molding compound. Epoxy is the most commonly used plastic molding compound for encapsulating semiconductor devices. Recent improvements in the techniques for epoxy encapsulation of semiconductor devices have made this method a serious competitor of the more costly hermetic sealed packages for semiconductors.
However, a disadvantage of epoxy encapsulation is that internal visual inspection of the encapsulated device is extremely difficult once the chip has been molded in plastic. However, internal inspection is commonly required of a sample device from a deliverable lot for quality and workmanship assessment and for failure analysis of failed devices. Internal inspection of epoxy encapsulated devices is not possible unless the epoxy which covers the chip and bonding wires is removed. The so-called decapsulation should, of course, not destroy the chip and bonding system and subsequent electrical testing must remain possible.
In the prior art, various methods are employed for decapsulation of epoxy-packaged devices. The most commonly used solvents are hot concentrated acids such as sulfuric acid and nitric acid. Commercially available solvents of a proprietary composition are sometimes useful for certain types of epoxy and the plastic can also be ashed in an oxygen plasma. However, all of these methods have their own specific disadvantages; some cause considerable damage to the package and others may take a very long time to uncover the chip.
When an epoxy-packaged integrated circuit is dissolved by immersion in hot sulfuric acid, rapid contamination of the acid will occur, resulting in difficulty of removing the decapsulation debris from the chip afterwards. Where routine decapsulation of samples is being done, this means that frequent replacement of the etchant and large quantities of hot acid are required, which from a safety point of view is rather undesirable. In practice, this method is therefore feasible only on a small scale, e.g. for occasional failure analysis. Complete immersion also often causes severe damage to the package which can make further electrical testing impossible.
Another disadvantage of liquid solvents is that unless the decapsulation progress is closely monitored and the time involved kept to a minimum, there is a risk of damage to chip metalization or to the bonding system. This is especially unsuitable since the two most frequently reported defects in epoxy-packaged semiconductor devices are corrosion of chip metalization and hot intermittent opens of the bonding wires.
A jet-etch technique is discussed in an article entitled "Jet-Etching Plastic Encapsulations," published by Electrical Quality Assurance Directorate, Procurement Executive, Ministry of Defense, "Aquila," Bromley, Kent, BR1 2JB. This article describes a decapsulation method where hot sulfuric acid is forced from a pressurized stainless steel container through a nozzle and sprays the package which is clamped on top of the container, thus producing a hole towards the silicon chip. However, this method results in rapid contamination of the acid due to the use of stainless steel for the container and the necessary overpressure and dispose of used etchant and debris form a potential safety hazard. Moreover, a clamping system is required to keep the package in place.